This invention relates to a wireless communication method and wireless base station, and more particularly to a wireless base station and wireless communication method that performs data communication using sub-carriers.
In a Digital Terrestrial Television system or an OFDM communication system that uses OFDM (Orthogonal Frequency-Division Multiplexing), measurement of the receiving power of the signal, controlling the receiving power, and channel estimation are performed using a time-division multiplexed common pilot signal as the transmission signal.
FIG. 29 is a drawing showing the construction of a transmission apparatus in an OFDM communication system, where a data-modulation unit 1 modulates the transmission data (user data or control data) using QPSK modulation, and converts the transmission data to a plurality of baseband signals (symbols) having an in-phase component and quadrature component. A time-division-multiplexing unit 2 performs time-division multiplexing of the pilots of a plurality of symbols in front of the data symbols. A serial-to-parallel converter 3 converts input data to parallel data of M symbols, and outputs M sub-carrier samples S0 to SM-1. An IFFT (Inverse Fast Fourier Transform) unit 4 performs IFFT (inverse fast Fourier transformation) processing of the sub-carrier samples S0 to SM-1 that were input in parallel and combines them to output a discrete-time signal (OFDM signal). A guard-interval insertion unit 5 inserts a guard interval into the M symbol long OFDM signal that was input from the IFFT unit 4, and a transmission unit (TX) 6 performs DA conversion of the OFDM signal inserted with a guard interval, and converts the frequency of the OFDM signal from baseband to the wireless band, then performs high-frequency amplification and transmits the signal from an antenna 7.
FIG. 30 is a drawing explaining the serial-to-parallel conversion, where a common pilot P is time multiplexed in front of one frame of transmission data. In the case where the common pilot per frame is 4×M symbols, and the transmission data is 28×M symbols, the M symbols of the pilots are output from the serial-to-parallel converter 3 the first four times as parallel data, and after that the M symbols of the transmission data are output 28 times as parallel data. As a result, the pilots in one frame period can be time multiplexed on all of the sub carriers and transmitted 4 times, and on the receiving side the channel is estimated for each sub carrier using those pilots, and channel compensation (fading compensation) becomes possible. An OFDM symbol is configured with M symbols.
FIG. 31 is a drawing explaining the insertion of guard intervals. Guard-interval insertion is copying the end section to the start section of an IFFT output signal that corresponds to M sub-carrier samples (=1 OFDM symbol). By inserting guard intervals GI, it is possible to do away with the effect of symbol interference due to a multipath.
FIG. 32 is a drawing showing the construction of an OFDM receiving apparatus. The signal that is output from the transmission antenna 7 is received by the receiving antenna 8 of the receiving apparatus via a fading propagation path, and a receiving circuit (Rx) 9 converts the RF signal that was received from the antenna to a baseband signal, and performs AD conversion to convert that baseband signal to a digital signal, then an FFT timing synchronization circuit 10, which extracts the signal of a desired band from the signal after AD conversion, detects the FFT timing from a time domain signal that includes the signal of the desired band that is output from the receiving circuit 9, and a symbol-extraction unit 11 extracts the OFDM symbols at FFT timing and inputs them to an FFT unit 12. The FFT unit 12 performs FFT (fast Fourier transformation) processing for each extracted OFDM symbol, and converts the signal to frequency domain sub-carrier samples S0′ to SM-1′. By calculating the correlation between the pilot symbols that were received during a set interval and a pre-known pilot pattern, a channel-estimation circuit 13 estimates the channel for each sub carrier, and a channel-compensation circuit 14 uses the estimated channel value to compensate for channel fluctuations of the data symbols. By the above processing, transmission data that is distributed by each sub carrier is demodulated. After that, the demodulated sub-carrier signals (not shown in the figure) are converted to serial data, and decoded. The example above is a process in which pilots are used in channel estimation, however, they can also be used in measuring the received signal power, SN ratio or the like.
As shown in FIG. 33, when pilot symbols exist at only the start and/or end of a frame, the receiving power of the data between pilots is estimated by the receiving power of the pilots. As shown by the solid line A in FIG. 34, when the speed of movement of a mobile station is slow, for example, walking speed (approximately 4 km/h), the time-variation interval of the received electric field intensity E becomes long, the variation width becomes small, and sudden drops decrease, so it is easy to estimate the receiving power of the pilot symbols. However, as shown by the dashed line B in FIG. 34, when the speed of movement of a terminal is fast, the time-variation interval becomes short and the variation width becomes large. Furthermore, it becomes easy for sudden drops to occur. As a result, the precision of estimating the receiving power between pilot symbols decreases. Also, the channel estimation precision decreases, and since decoding and demodulation are performed using these poor estimation results, the quality of communication decreases. In other words, when moving at high speed, the channel estimation precision decreases and thus the communication quality and throughput decrease. A detailed case will be explained below.
A case is presumed in which communication is performed with a base station having 100 terminals inside a cell, where 50 of the terminals are moving at high speed, and the remaining 50 terminals are moving at low speed or are still. The channel estimation precision of the terminals moving at high speed decreases, as well as the quality of communication decreases and the transmission speed decreases. Here, supposing that it is not possible to maintain the required communication quality for 25 of the high speed terminals, and the transmission speed becomes 0, the overall transmission speed throughput of the base station becomes 0.75. Hereupon it is assumed that the throughput is 1 when all of the terminals are moving at low speed or are still. In this way, when the interval between pilot symbols is long, the communication quality of terminals moving at high speed decreases, and the overall transmission speed (throughput) of the base station decreases.
First Prior Art
In regards to the problem described above, a method is feasible, as shown in FIG. 35, in which the interval between common pilot symbols is narrowed, and the number of pilot symbols is increased. However, in this method the following problems exist, so this method is not desirable.
(1) The pilot symbols are common pilot symbols, so the number of symbols increases regardless of whether or not the terminal is moving at high speed.
(2) Data is decreased by the amount that pilot symbols are added, so the actual transmission speed decreases.
A detailed example will be explained below.
Supposing that the overall transmission speed of a wireless frame is 10 Mbps. Then here, the ratio between the normal number of pilot symbols and the number of data symbols is taken to be 0.1:0.7. The remaining 0.2 is the control signal. Therefore, the actual transmission speed is 7 Mbps.
Next, as countermeasures for high-speed movement, the interval between pilot symbols is narrowed, and 2× the number of pilot symbols are inserted. By doing this, the aforementioned ratio becomes 0.2:0.6, and the actual transmission speed drops to 6 Mbps. Therefore, even though some terminals may be moving at low speed, when pilots are added and inserted as a countermeasure for high-speed movement, the actual transmission speed becomes 1 Mbps (15%) less than 7 Mbps. As described above, by simply just adding pilot symbols, a drop in actual transmission speed, as well as a decrease in throughput occurs. Therefore, the overall transmission speed (throughput) of the base station decreases.
To handle this problem, a method has been proposed that performs variable control of the number pilot symbols according to the propagation environment (see JP 2000-151548A and JP 2005-027294A). This method measures the propagation environment, and performs control so that when the propagation environment is poor, it increases the number of pilots, and when it is good, decreases the number of pilots. However, the number of symbols is increased or decreased for each individual mobile station, so control is complicated. Particularly, there is a problem in that since the number of symbols is increased or decreased for each individual mobile station, scheduling control becomes difficult.
Second Prior Art
Also, in regards to terminals moving at high speed, as shown in (a) of FIG. 36, a method has been proposed in which, in addition to common pilots P, dedicated pilots PD are added between the common pilots (see JP 2001-197037A). However, in this method of inserting dedicated pilots, control must be performed of measuring the propagation environment and inserting dedicated pilots when the environment is poor, and not inserting pilots when the environment is good. Therefore, as in the first prior art, control of each individual mobile terminal is necessary, and there is a problem in that together with the control being complicated, scheduling control becomes difficult. Also, the insertion position where the dedicated pilots are inserted is treated as a special position, and when dedicated pilots are not inserted, then as shown in (b) of FIG. 36, a special control signal is inserted, thus data decreases and transmission speed drops. In other words, for terminals moving at low speed, data cannot be inserted in the positions where the dedicated pilots are inserted, so as a result, the same problem occurs as in the case when the interval between pilot symbols is narrowed and the number of pilot symbols is increased.